The present invention relates to frame buffer memory systems for raster displays, and more particularly to a frame buffer memory permitting rapid picture updating and rapid read-modify-write oprations.
Raster scan frame buffer displays have become increasingly popular as the price of semiconductor memory has decreased. The image to be displayed is represented in a large memory that saves a digital representation of the intensity and/or color of each picture element, or pixel, on the screen. By properly recording the data in the memory an arbitrary image can be displayed, making the display hardware insensitive to image content. The frame buffer memory is equipped with hardware to generate a video signal to refresh the display and with a memory port to allow a host computer or display processor to change the frame buffer memory in order to change the image being displayed.
Interactive graphics applications require rapid changes to the frame buffer memory. Although the speed of the display processor is clearly important to high performance, so also are the properties of the memory system, such as update bandwidth, i.e., the rate at which the data processor may access the frame buffer memory. For a given memory technology the implicit geometry of frame buffer memory access can affect this rate.
In conventional frame buffer memories, when a new image is to be combined with an existing image in some way, as for instance, when the new image is to be superimposed over the existing display image, the existing image data is read and transmitted to the host processor, which combines it with the new image data in the appropriate manner. The result is then written into the frame buffer memory. This operation requires a memory read and a memory write cycle along with whatever processor cycles are necessary to perform the pixel combination logic.
What is desired is a means for providing flexability in the way pixel data is read from or written to a frame buffer memory and for speeding up the process of updating the image in frame buffer memory during a read-modify-write operation.